This invention relates to linear velocity measuring sensors. In particular, but not exclusively, this invention is a device that allows gun-fired, smart ammunition to precisely measure its own muzzle exit velocity for use in precision guidance. This invention allows for the precise measurement of velocity in the axial direction of motion, while cancelling the effects of motion in the radial direction.
Sensors are generally used in smart ammunition in order to improve precision, especially at long firing ranges. The ultimate goal is to reduce target impact dispersior of a particular round. While in flight, a smart round will use its sensors to determine if corrections are necessary to its trajectory. For instance, GPS modules, magnetometers, and gyroscopes, can be used to determine location, heading, and orientation respectively. The round's fuze will use that information to determine if maneuver needs to be performed, if the round should recalculate a timer, if the round should dud, and the like.
It is sometimes necessary for a smart round to know its muzzle exit velocity. In smart rounds that use a timed detonation—such as those that have an air burst mode—the time set for detonation is computed from: the range to target, the expected trajectory of the round, and an expected muzzle exit velocity. In order for the round to precisely detonate at a specific point in its trajectory, especially for air burst mode, all of these factors need to be precisely known.
Muzzle exit velocity is typically a value that is not measured, rather it is estimated before the round is shot based on the type of round being fired, its weight, and the type and amount of propellant being used. This is contrary to the range to target and round trajectory, both of which are already accurately determined by laser range finder and ballistic computer, respectively.
While certain methods have been proposed to measure muzzle velocity, each has certain shortcomings. For example, in “Apparatus for Measure the Muzzle Velocity of a Projectile” (U.S. Pat. No. 3,659,201, issued April 1972), Remo Vogelsang describes an invention where two coils sit co-axially at the end of a gun tube, each energized with a DC current source, and connected electrical in series. When a bullet is fired, it generates two voltage pulses as it passes through both coils. The time difference between each pulse is measured, and combined with a temperature compensation technique for the spacing between each coil, velocity is determined.
In “Inductance Type Velocity Measuring Apparatus” (U.S. Pat. No. 3,824,463, issued July 1974), Kenneth L. Oehler describes a similar invention, where again, two coils sit co-axially at the end of a gun tube. In his invention, the both coils are electrically connected as frequency determining elements of an oscillator. When a bullet is fired through the coils, the frequency of the oscillator is modulated twice. An FM discriminator circuit then detects the modulation, subsequent pulse shaping circuits produce “start” and “stop” pulses, whose time interval between along with the spacing of the coils is used to calculate exit velocity.
Finally, in “Device and Method for Determining the Muzzle Velocity of Projectile” (US 2004/0250615 A1, Pub Date December 2004), Aldo Alberti and Klaus Munzel describe an invention where a single coil is placed co-axially at the end of a gun tube, energized with a DC current, creating small localized magnetic field. As a bullet is fired, it changes the magnetic field within the coil, which induces a voltage pulse across the terminals of the coil. This pulse is detected, its pulse width measured, and used to determine exit velocity.
Nonetheless, there remains a need for a more accurate muzzle velocity measuring device as provided by this invention.